The present invention relates generally to a composition and method for shielding radioactivity and, more particularly, to a modified urethane/phenolic resin polymer composition shielding material capable of encapsulating and stabilizing nuclear waste within a polymeric matrix, and methods for using the polymer composition. Nuclear waste generated by the nuclear energy industry must be immobilized for safety and environmental reasons. The polymer composition of the present invention effectively immobilizes nuclear waste in the polymeric matrices safely and in a cost-effective manner. Further, when depleted uranium is the waste incorporated in the polymeric matrix, the polymer composition is a highly effective radiation shielding material that can be stored or utilized for secondary applications.
The tremendous growth in the nuclear energy industry demands the need for safe, cost effective immobilization of wastes generated. Nuclear power generation is dependent on enriched uranium-235 fuel. Given the natural abundance of the U-235 isotope is 0.7%, large quantities of uranium-238 isotope, known as depleted uranium, remain after processing. As there is very little demand for this isotope, U-238 is stockpiled.
One Department of Energy storage site in Paducah, Kentucky has 700,000 tons of depleted uranium in storage. Depleted uranium is a very low radiation source, but is very toxic and environmentally hazardous. Further, transportation to other storage sites is not feasible for economic and safety reasons.
In addition, the depleted uranium waste, which is stored underground, is often kept under heavy mineral oil to prevent atmospheric oxidation and subsequent combustion. The oil is known to be contaminated with polychlorinated biphenyls (PCBs), trichloroethylene (TCE), tetrachloroethane (TCA), and methylethylketone (2-propanone). This mixture cannot be treated by conventional stabilization techniques.
Treatment of radioactive waste is often further complicated by the presence of metal ions in the waste. Chemical fixation processes for containing hazardous waste containing metal ions have been accepted and used as viable alternatives to traditional remediation techniques, as these processes are relatively simple to implement and are cost effective. However, conventional methods of containment of the chemically immobilized waste have been shown to be ineffective.
Further, several isotopes that may be present in radioactive waste, such as Cs-137 and Sr-90 are of great value in treating diseases, sterilizing surgical tools, and food preservation. Therefore, an immobilization process that would allow the immobilization medium to be dissolved, if desired, to recover the radio nuclides therein is preferred.
Conventional stabilization/solidification techniques using cement grout have been successfully used to contain certain heavy metal contaminants, typically lead and mercury, but do not perform successfully when the total organic content exceeds about 3%. At high levels of organic compounds, which is common in the radioactive waste industry, any final product made with cement will remain in paste form, which is unacceptable under Emergency Response Disposal Facility (ERDF) criteria.
In addition, radioactive isotopes cause structural damage to cement based binders. Vitrification of the cement based binders provides a glassy matrix which is not self healing, increasing leaching problems.
In-situ vitrification technology requires metallic uranium to be converted into a uranium compound. Solubility limitations lead to large volume increases making this method economically infeasible. Further, vitrification provides only partial remediation of waste, typically about 8% of the clear liquids.
Solvent washing techniques also result in a very large volume of secondary waste streams, contaminated with radionuclides. Further, when the uranium has been completely washed of oily organics, the metallic uranium, which is pyrophoric, will spontaneously ignite. The final product therefore does not meet non-leachability and non-pyrophorocity ERDF criteria
In addition to treating/remediating nuclear waste, there is a great need for improved shielding materials, both for use in storing treated radioactive wastes and for the protection of personnel and instrumentation in facilities which utilize radioactive materials.
Conventional shielding materials include cement-based products and organic polymers. Cement products provide some protection, but have a low hydrogen atomic density, a measure of a materials"" ability to shield radiation. Specialty types of cement that incorporate an increased volume of water, resulting in higher hydrogen densities are available. Unfortunately, the amount of bonded water tends to decrease with time, causing a reduction in shielding ability, and the improvement in performance does not justify the increased expense.
Polyethylene is a commonly used polymer shielding material because of its high hydrogen atomic density, but requires a neutron absorbing material, usually boron. Polyethylene also has a low specific weight or density, and is typically used in combination with a layer of concrete or lead.
It is therefore an object of the present invention to provide a polymer material with improved shielding characteristics.
It is a further object of the present invention to provide a polymer composition and method that effectively immobilizes, both chemically and mechanically, radioactive waste materials.
Yet a further object of the present invention is to provide a polymer composition that incorporates depleted uranium to produce a moldable polymer composition which exhibits excellent radiation shielding properties.
It is another object of the present invention to provide a polymer composition that resists structural damage from heavy doses of radiation, primarily gamma radiation.
Yet another object of the present invention is to provide a polymer composition that produces excellent leaching characteristics after exposure to radioactivity.
A further object of the present invention is to provide a polymer composition that retains good mechanical properties after exposure to high level radiation.
Still a further object of the present invention is to provide a polymer composition that does not undergo biodegradation, corrosion in acidic environment, or exhibit damage due to weathering.
Yet a further object of the present invention is to provide a polymer composition that does not release combustible gas when exposed to high level radiation.
Another object of the present invention is to provide a polymer composition that will safely and economically store depleted uranium waste.
Still another object of the present invention is to provide a method of safely treating depleted uranium waste that has been stored under heavy mineral oil,
Yet another object of the present invention is to provide a method of treating radioactive waste that will remove all wastes, including solids, semi-solids, and organics to provide total remediation.
Yet a further object of the present invention is to provide improved containers for use in storing radioactive waste materials.
To accomplish the foregoing and other objects of the invention there are provided the following compositions and methods.
A urethane based polymer composition, composite, or blend is provided. The composition is formed by mixing a liquid isocyanate monomer and a liquid phenolic resin with a phosphate ester flame retardant at room temperature until a homogeneous mixture is formed. Presently preferred are diphenylmethane 4,4xe2x80x2-diisocyanate monomer and phenol formaldehyde resin. The presently preferred flame retardant is a halogenated phosphate ester. The resulting composition cures without heating in approximately 6-18 hours depending on environmental conditions.
A catalyst may be utilized in applications where a short curing time is necessary. Phenylpropyl pyridine is presently used as a catalyst, reducing the composition cure time to about 20 minutes, depending on environmental conditions.
In a preferred embodiment of the present invention the polymer further incorporates radioactive waste. The solutions of monomers, including diphenylmethane 4,4xe2x80x2-diisocyanate and phenol aldehyde resin are mixed at room temperature with the waste to be immobilized until a homogeneous mixture is formed. The resulting polymer/waste composition cures in 6-8 hours depending on environmental conditions. The resulting composition does not deteriorate or suffer structural damage when exposed to higher levels of gamma radiation, nor do the mechanical or chemical properties undergo any detectable change. The composition is resistant to biodegradation and combustion, and does not creep or shrink during thermal cycling.
In one of the preferred embodiments, the waste that is encapsulated into the polymer matrix is depleted uranium. In a preferred composition, a mixture of about 6% of the liquid monomer defined above and about 94% depleted uranium in powder form is mixed until homogeneous. The mixture is then allowed to cure at room temperature. The cured polymer composition provides excellent radiation shielding characteristics, without damage to the polymer matrix or leaching of the radioactive material. When depleted uranium or other low level radioactive material is incorporated, this composition solves two problems. The composition deals with a problematic waste material and provides a useful product.
When the waste to be mixed with the polymer of the present invention contains metal ions, chemical fixation to prevent leaching is necessary. The governing properties of an effective chemical immobilization have been shown to include the solubility product constant and the partition coefficient of the final waste form. The anions, selected based on Ksp (solubility product constant), are mixed with the waste to affect chemical fixation prior to the waste being mixed with the liquid monomers.
The shielding composition may be utilized in several manners. The monomers can be mixed and the resulting solution sprayed or poured, prior to curing, over the radioactive material, in order to contain the material and prevent leakage of radiation. In addition, the uncured composition can be sprayed on the walls of a room or container to prevent leakage of radiation, and can also be used to contain radiation prior to demolition. Further, the polymer composition can be molded to produce bricks or panels that may be utilized as part of the construction of a containment vessel or room.
In a preferred use, the polymer is utilized in a thermal desorption/polymer based immobilization method to safely deal with U-238 stored under contaminated mineral oil. In this process, the oil and other organic material is distilled off at reduced pressure in the presence of the inert gas helium. The distilled oil and vapors are combusted in a high efficiency burner. The combustion products are further oxidized by a series of catalytic oxidizers. The off-gases are scrubbed chemically, filtered and the helium is recycled. The uranium metal is cooled in the helium atmosphere and then immobilized with the radiation shielding polymer of the present invention. Fire hazards are eliminated as all the processes are conducted in a helium or flame retardant atmosphere at room temperature.
A preferred construction using the polymer composition as an integral part of a radiation containment vessel is a double walled stainless steel container. The void between the two walls is filled with the polymer composition material. The containers include a solar photo electric cell panel which will provide electricity for cooling fans to circulate air and dissipate any heat produced. Corrosion is prevented by the use of sacrificial electrodes, which are attached to each container.